Optimizing Drilling Mud Shearing

ABSTRACT

Viscosity and other properties are determined at desired temperatures in drilling mud and other fluids by using a versatile cavitation device which, operating in the cavitation mode, mixes and heats the fluid to a specified temperature, and, operating in the shear mode, acts as a spindle for application of Couette principles to determine viscosity as a function of shear stress and shear rate. The invention obviates the practice of adjusting rheology of a drilling fluid by passing it through the drill bit. Drilling fluid may be managed by a “straight-through” method to the well, or by placing the cavitation device in a loop which isolates an aliquot of known volume and circulating the fluid through the loop including the cavitation device. A controller may be programmed to manage the viscosity and other properties at various temperatures by controlling the power input and angular rotation of the “spindle” (which has cavities on its cylindrical surface), and feeding viscosity-adjusting agents and other additives to the fluid. Data may be collected from the loop and used in the “straight-through” mode until it is determined that conditions require a new set of data, or the loop may be used continuously. The system may be used with a supplemental viscometer, density meter, and other instruments.

FIELD OF THE INVENTION

This invention relates to the preparation and maintenance of drillingmud properties, especially rheology, viscosity, shear, density, solids,and water/oil ratio, either at a mud plant where drilling fluids aremixed and stored for delivery or during the ongoing drilling process inthe recovery of hydrocarbons from the earth. The invention utilizes acavitation mixer placed in a loop capable of isolating an aliquot ofdrilling mud from either the conduit leading to the well or a tank usedto make up new mud or to store mud. The cavitation mixer imitates theshear and heating generated when pumping the drilling mud through thedrilling bit at various temperatures. The rheology and other propertiesof the drilling mud are maintained at desired values by regulating, inthe loop, the addition of viscosity-adjusting agents, other additives,the flow rate, and the speed of the mixer to obtain a desired shearingeffect in the mixer in real time without the need for lab tests. Thecavitation mixer not only heats and shear mixes, but is able to functionas a viscometer, reinforcing optional separate viscometer readings.Other properties can be monitored and regulated in the loop.

BACKGROUND OF THE INVENTION

Drilling muds are complex, typically non-Newtonian fluids that servemultiple, critical functions in drilling wells for oil and gasextraction. The fluid is used to remove formation drill cuttings fromthe wellbore, and the fluid adds hydrostatic mass to help preventuncontrolled flow of hydrocarbons from the well. The fluid also enablesbuoyancy to counteract the weight of the drill pipe so that one candrill deeper wells. The fluid also lubricates the bit and stabilizes thewellbore as drilling continues deeper. Limiting the loss of fluid to therecently drilled formation is another important function, and limitingfluid loss typically means the use of bridging agents that are sizedparticles. It is essential to know the properties of the fluids so theycan perform their many functions efficiently.

Some fluid properties are relatively easy to determine in line as thefluid is being used. For example, a Coriolis Meter can accuratelydetermine the flow rate and density of the fluid, but determiningrheology of a complex drilling fluid is more complicated; it is commonlydone by a manual mud check according to API 13B. Accurate knowledge of afluid's rheology is required to calculate a Yield Point and plasticviscosity. If the mud is too thick, the mud pump cannot pump it. If thefluid is too thin, it may not suspend the solids that have to be removedfrom the wellbore as one continues drilling deeper. To continue drillingdeeper, drill pipe has to be added to the string. During additions tothe string, the mud is no longer being pumped, and Yield Point, which ispart of the rheology, determines the pressure needed to move the fluidagain after it has been static in the wellbore. If Yield Point (YP) istoo high, the pump cannot begin to move the flow of mud.

Prior to the present invention, it has been common not to attempt toshear mix a drilling mud before it is sent down the well to the drillbit, but rather to utilize the drill bit itself to shear mix thedrilling mud. This means the rheological properties of the drilling mudare not the most desirable when the mud arrives at the point ofdrilling, and often can be far from optimum. Moreover, the drillingprocess adds drill cuttings and other solids and fluids to the drillingfluid, which continuously change significantly the physical propertiesof the fluid. The prior method, relying on the drill bit for shearmixing, injects considerable uncertainty into the overall process.

One reason the art has relied on the drill bit for shear mixing is thatthere had not been available a practical way to shear mix the ponderousdrilling mud components in a continuous recycling mode.

It takes time to run the “mud checks” specified by the AmericanPetroleum Institute (API) 13B. Mud checks require a skilled operator tosuccessfully run and to report the mud properties. Without shearing themud, however, the chemistry is not fully activated and the desiredrheology is not achieved. In the laboratory either a Hamilton Beachblender or a Silverson mixer is used to imitate the shear developed by atrip through the drilling bit. There is disagreement about which deviceto use and the amount of time required to mix the mud before running amud check. Both the Hamilton Beach blender and the Silverson havecommercial units that replicate their laboratory units, but they are nottypically used for large batches at a drill site for a number ofreasons. One problem is simply time. Typically in the laboratory it iscommon to make 350 ml portions to represent one barrel of fluid. If youshear a one barrel equivalent drilling fluid sample in the laboratoryfor 5 minutes, then to “scale up” the shear process at the wellsite, ittakes the same 5 minutes per actual barrel. Unfortunately it takes toomuch time. If a rig has 1,000 bbls of drilling fluid, it would take5,000 minutes or 3.5 days of processing to equal 5 minutes of shear usedin the laboratory for the 1 bbl equivalent volume.

Volumes of drilling mud can range from 500 bbls to over 10,000 bbls onlocation that is stored in pits or tanks, and the mud can stratify basedon the density of the additives. When relying on samples for API 13B, itis critical that they are representative of the drilling fluid to beused in the well, but all too often, imperfect sampling practiceintroduces errors into the API 13B procedure.

Rheology of drilling muds is measured using a Fann 35 or equivalentrotational viscometer that directly reads viscosity on a dial atdifferent rpm. The dial reading is based on the deflection of a bobinside of a rotating cylinder, and the instrument must be calibratedregularly to be accurate. Temperature changes mud rheology, and todetermine an accurate downhole rheology means the mud must be heatedbefore measuring its viscosity. By definition, a mud check is done“offline” which takes valuable time and can delay critical decisionsabout well control. Rig time is often lost while the fluid is circulatedin the hole to adjust drilling fluid for the proper rheology after atime delay and before continuing to drill.

A better way to conduct the shear mixing and the rheology measurementprocess is needed. Ideally a realtime, inline measurement of the mudproperties is desired, but there are several challenges to itsachievement. One challenge is simply the shear that happens at the bitneeds to be replicated at the surface. There are high-pressure mixingdevices that accomplish this shear, but they are expensive to build andoperate; moreover, high-pressure is an HSE (health, safety andenvironmental) issue. The rheology measuring device is anotherchallenge. Rheology measurement is used in numerous industries and thereare a number of devices adapted for oil field use that include, but arenot limited to, the Brookfield TT-100, Grace M3900, and Chandler 3300.The advantage of these types of devices is that they can be calibratedto replicate the Fann 35, and Fann 35 readings have become a de factostandard and it is not uncommon for mud engineers to quote viscosity atvarious Fann 35 speeds, or add chemistry based on a particular Fann 35reading. A Fann 35 is a Couette style viscometer as are these threedevices, and while they can be correlated to Fann 35 readings, they haveintricate internal parts and small flow lines that can easily plug whenfluid loss additives are in the drilling fluid. There are numerous otherviscometers used in other industries that presumably would also work;however, a viscosity measurement is taken at a single shear rate or atshear rates that are harder to relate to a Fann 35 viscosity reading.

Rheology requires a shear rate vs shear stress curve to accuratelycalculate plastic viscosity and yield point. A pipe rheometer can beused to measure viscosity by accurately measuring the pressure dropacross a known length of pipe of a known internal dimension whilemeasuring an accurate flow rate. Pipe rheometers are commerciallyavailable from Chandler Engineering, Stim-Lab, Inc, or Khrone but theyare relatively simple devices that can easily be built assuming anunderstanding of flow and viscosity calculations that are widelypublished. An example of the calculation required has been published byPetroleum Department of The New Mexico Institute of Mining andTechnology as a class exercise available on the Internet as“L5_PipeViscometer.pdf”.

An ideal device to measure flow in a pipe rheometer is a Coriolis meterwhich has a full opening pipe internal diameter such that it is noteasily plugged. A Coriolis meter also gives an accurate mass flow, notjust a volume flow rate. Coriolis meters such as the E+H Promass 83I canalso measure viscosity. Given the critical performance required ofdrilling muds to ultimately prevent uncontrolled well events, using acombination of rheology measurement devices based on differentprinciples would make sense. For example, a pipe rheometer requiresaccurate flow rate. Using the E+H Promass 83I for accurate flow ratecould also validate the viscosity being reported by pressure drop.Whereas the pipe rheometer calculations are based on flow and pressuredrop, the Promass 83I viscosity is a function of a vibration frequency.

Even with otherwise proper rheology measurement techniques, heat is anadditional challenge. The fluid rheology should be measured at more thanone temperature. Therefore the ideal device would shear the mud toreplicate the shear imparted by the drill bit, heat the fluid to theproper temperature and report rheology at different predeterminedtemperatures.

Another challenge is where to take a sample. Drilling fluid oftenstratifies in a tank. A sample taken at the top of the tank, or at anysingle level, will not be representative of the composition in theentire tank.

SUMMARY OF THE INVENTION

Drilling mud is monitored and adjusted with immediate response torequirements by placing a cavitation mixer in a loop on the conduitleading from the source of mud ingredients to the well. The loop canisolate an aliquot of the mud to be used so that its rheology,viscosity, density and other properties can be determined at known flowrates and at temperatures present around the drill bit, and adjustedaccordingly. Lab tests are not needed.

A cavitation mixer is a cavitation device used for mixing and heatingfluids; in the present invention, it is also used to determine rheologyof the drilling mud.

The phenomenon of cavitation, as it sometimes happens in pumps, isgenerally undesirable, as it can cause choking of the pump and sometimesconsiderable damage not only to the pump but also auxiliary equipment.However, cavitation, more narrowly defined because it is deliberatelycreated, has been put to use as a source of energy that can be impartedto liquids. Certain devices employ cavities machined into a rotorturning within a cylindrical housing leaving a restricted space forfluid to pass. A motor or other source of turning power is required. Thephenomenon of cavitation is caused by the passage of the fluid over therapidly turning cavities, which creates a vacuum in them, tending tovaporize the liquid; the vacuum is immediately filled again by the fluidand very soon recreated by the centrifugal movement of the liquid,causing extreme turbulence in the cavities, further causing heat energyto be imparted into the liquid. Liquids can be simultaneously heated andmixed efficiently with such a device. Also, although the cavitationtechnique is locally violent, the process is low-impact compared tocentrifugal pumps and pumps employing impellers, and therefore as amixing technique is far less likely to cause damage to sensitivepolymers used in oilfield fluids. Good mixing is especially important inmixing drilling muds.

Examples of cavitation devices are described in U.S. Pat. Nos.5,385,298, 5,957,122, 6,627,784 and particularly 5,188,090, all of whichare hereby specifically incorporated herein by reference in theirentireties. These patents may be referred to below as the HDI patents.

The basic design of the cavitation devices described in the HDI patentscomprises a cylindrical rotor having a plurality of cavities bored orotherwise placed on its cylindrical surface. The rotor turns within aclosely proximate cylindrical housing, permitting a specified,relatively small, space or gap between the rotor and the housing. Fluidusually enters at the face or end of the rotor, flows toward the outersurface, and enters the space between the concentric cylindricalsurfaces of the rotor and the housing. While the rotor is turning, thefluid continues to flow within its confined space toward the exit at theother side of the rotor, but it encounters the cavities as it goes.Flowing fluid tends to fill the cavities, but is immediately expelledfrom them by the centrifugal force of the spinning rotor. This creates asmall volume of very low pressure within the cavities, again drawing thefluid into them, to implode or cavitate. This controlled, semi-violentaction of micro cavitation brings about a desired conversion of kineticand mechanical energy to thermal energy, elevating the temperature ofthe fluid without the use of a conventional heat transfer surface.

I refer to the cavitation device I use as a cavitation mixer because itis sometimes, in my invention, used as a shearing device instead ofheating by cavitation, as will be explained below. The loop in which thecavitation mixer is placed will also be described and explained below.

The ingredients for a drilling mud are placed in a mud tank or othercontainer and may be roughly mixed together in any conventional manner.As they are withdrawn to be sent down the well, they encounter acavitation mixer, preferably a flow-controlled cavitation mixer,referred to herein as a FCCM. The preferred FCCM is a TrueMud™ mixer.The FCCM has variable mixing rates based on the speed of the disc(rotor) and the rate that fluid is pumped through the device, is able totake in additives at controlled rates or dosages, assures a uniform andturbulent entry, preheats the fluid before beginning the cavitationprocess, and includes means for setting the gap in the entryway as afunction of the viscosity of the fluid. The drilling mud passescontinuously through the FCCM to the well, where it is destined for thedrilling bit. On its way to the well, a rheology meter or viscositymeter reads its rheology or viscosity directly in the conduit, or fromsamples taken from it, or on a bypass line.

A cavitation device comprises a cavitation rotor within a housing. Thecylindrical surface of cavitation rotor has a large number of cavitiesin it. Its housing has a cylindrical internal surface substantiallyconcentric with the cavitation rotor. The cavitation rotor is mounted ona shaft turned by a motor. Fluid entering through an inlet spreads tothe space between the cavities and the conforming cylindrical internalsurface of the housing and is subjected to cavitation—that is, it tendsto fall into the cavities but is immediately ejected from them bycentrifugal force, which causes a partial vacuum in the cavities; thevacuum is immediately filled, accompanied by the generation of heat andviolent motion in and around the cavities. This highly turbulent actionin the cavitation zone between the two cylindrical surfaces of thecavitation device thoroughly mixes and heats the materials before themixture passes through an outlet. While any cavitation device as justdescribed may perform satisfactorily in my invention, I prefer to use a“flow-controlled” cavitation device, which has a generally conicalsurface positioned centrally on the rotor to face the incoming fluid andto assist the flow of the fluid to the perimeter of the rotor.

The rheology or viscosity meter combined with the FCCM mixer eliminatesthe need for routine mud checks by continuously reporting rheology usinga process control loop. Mud checks primarily report rheology bymeasuring shear rate and shear stress at predetermined temperatures. Inthe prior art, typically a sample of the fluid is heated to 100° F.where viscosity measurements are taken with a Fann 35 or equivalentrotor and bob viscometer and then the measurements are repeated at 150°F. This is a time-consuming procedure which delays reports, oftenresulting in their misapplication. The TrueMud™ mixer used in thepresent invention is a heating device that not only shears the mud, butheats it by converting shaft horsepower into heat. By adding temperaturecontrols to the device, the heat can be adjusted with the speed (RPM, orangular velocity) of the disk and the flow through the device. By addinga pump and a bypass line, a process control loop with a known volume canbe circulated in the present invention to shear the mud at a given flowrate and to heat the mud to report accurate, temperature dependentrheology of the fluid actually headed to the well.

There are several devices that can measure viscosity of drilling mudincluding, but not limited to a Brookfield TT-100 that measuresviscosity at different reciprocal seconds of shear to provide real timerheology. Other devices such as an Endress+Hauser Promass 83i measuresviscosity based on vibration feedback correlated to different reciprocalseconds. Since drilling muds contain solids, a pipe rheometer also workswell, also enabling the calculation of a friction factor at differentflow rates. A pipe rheometer calculates rheology based on flow rate andpressure drop across a known length of pipe. Once steady-state isreached in the process control loop, fluid can be added and removed fromthe same loop by controlling the valves that allow fluid into and out ofthe process control loop. The flow into and out of the loop can beautomatically adjusted by simple temperature, desired process flowrateor by viscosity/rheology measurement. Using all digital sensors, theprocess control loop can be automated by a process logic controller,and/or a computer and then easily reported remotely using the Internet.Such a device including the process control loop is scalable. A 1 inchdevice may be used simply as a method to do automated mud checks or as alaboratory device, whereas a 2 inch or 3 inch device can fully processthe fluid being used at the well.

The combined shearing device and rheology process control loop acts on arealtime aliquot of the drilling fluid in the well or in a tank. Therealtime aliquot solves the problem of sampling in a stratified tank, orrunning mud checks in a dynamic system where the mud is changing for anynumber of reasons including water influx. The known volume of fluid inthe loop is actually used in the well, unlike a sample tested in alaboratory. The aliquot may be isolated and the method of the inventionperformed while drilling is stopped (for example when a pipe is added tothe drill string) and the mud is not flowing to the well, or whiledrilling is progressing.

The process control loop can contain (contains) mixing equipment suchthat the realtime aliquot of the larger volume of drilling mud can beadjusted and rheology, density, pH, electrical stability, and otherproperties measured immediately to check potential mud treatments beforemixing the full volume of drilling fluid. In the laboratory suchadjustments are done on a “barrel equivalent” of mud which consists of350 ml of fluid and 1 gram equals 1 pound per barrel. In the field youhave to adjust the full mud volume and wait to run mud checks after themud has circulated through the drill bit. That process often takes morethan one circulation and more than one mud check to get the desired mudproperties. There is a known volume of mud in the process control loopof the present invention and a known flow rate. Small volumes ofchemistry can be mixed into the mud either by isolating the processcontrol loop or by proportioning the chemical concentration based onflow in the process control loop to immediately know the mud propertiesbefore adding the chemistry (chemicals) to the full volume of mud.

Furthermore, the meter can send readings continuously or intermittentlyto a controller which controls the addition of mud thinner, polymers toadd viscosity, and other additives. The controller also controls thespeed of the mixer and/or flow through the mixer. The thus prepareddrilling mud proceeds to the drill bit where it already has the desiredattributes.

In the vicinity of the drill bit, the drilling mud picks up drillcuttings and other solids; the drilling mud is designed to do soefficiently and carry the solids back to the surface where the bulkcuttings are removed with surface separation equipment such ashydrocyclones, screens and centrifuges. The separation process isdesigned to be efficient, but some of the mud is lost in this processand low gravity solids below 10 micron in size are generally notremoved. New mud is mixed in a mud tank where the used mud including lowgravity solids that could not be removed is mixed with new drilling mudingredients, thus changing the properties of the material in the tank.The invention continues to adjust the properties of the drilling mud bymonitoring and maintaining viscosity or rheology by regulating theenergy input to the FCCD and the amount of additives replenished oradded to the drilling mud. Furthermore information generated in thepresent invention can be used to remotely monitor the drilling mud suchthat a skilled mud engineer is not required to do continual mud checksat the rig site and can therefore manage more rigs.

The TrueMud™ (FCCD) mixer configuration allows for viscositymeasurements since it is essentially a Couette style device with arotor. The calculations are widely reported in the literature and can befound on Page 21 of “More Solutions to Sticky Problems” published byBrookfield Engineering Labs, Inc.

The FCCD is a spinning disk inside of a cylinder and can be set up tomeasure rheology. Rheology is shear stress measured at different shearrates. Shear rate is represented by the speed of the spinning disk.Shear stress is the torque required to spin the disk. Both can beaccurately measured and the calculations are known to convert the speedof the disk and torque into a viscosity. There are some unknowns such ascritical velocity. To measure rheology, the fluid should be in laminarflow below the critical velocity. To overcome any unknowns, the FCCD canbe calibrated either by using a calibration fluid such as 100 centistokesilicone oil that is also used to calibrate laboratory rheometers, orthe viscosity can be normalized to one known viscosity point usingpressure drop across a known length of pipe or a device that measuresviscosity at one shear rate. Furthermore, the viscosity can be comparedto a manual Fann 35 reading done in the field and in all cases softwarecan be used to adjust the viscosity to match the Fann 35 viscosity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the prior art method using the drill bit to shearthe fluid.

FIG. 2 is a diagram of the invention method.

FIG. 3 is a partly sectional view of the flow controlled cavitationmixer.

FIG. 4 shows a basic process loop including a cavitation mixer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates diagrammatically the prior art method of relying onthe drill bit to shear mix the mud ingredients. The parts are not shownin relative proportion. Mud tank 1 contains the ingredients for adrilling mud. It may have a rough mixing capability, not shown. Asdrilling commences and proceeds, the mud in tank 1 is sent in conduit 2to the well 3 below rig 6, following the path indicated by thedownwardly oriented arrows to the bottom of the well 3 and the drill bit4. The fluid may be directed through nozzles or ports on the drill bit,causing shearing. As the drill bit 4 does its work, drill cuttings arecreated, and these are picked up by the drilling mud and removed asindicated by the upwardly oriented arrows. From the top of the well 3,the solids-laden used drilling fluid is returned through conduit 5 tothe tank 1 where it mingles with the mud ingredients already there. Theeffects of shearing through or around the drill bit are difficult torelate to the properties of the fluid in the tank. Moreover, the fluidis not sheared prior to entering the well, as is desirable. In addition,the prior art method, and modifications of it, rely on time-consumingand error-prone sampling and laboratory tests.

Referring to the simplified diagram of the invention in FIG. 2, mud tank11 contains the ingredients for a drilling mud. It normally will have arough mixing capability, not shown. As drilling commences and proceeds,the mud in tank 11 is sent in conduit 12 to the flow-controlledcavitation mixer 17, where it is shear mixed, and then through conduit18 to viscometer 19, which measures its viscosity. It then continues inconduit 18 to well 13 associated with rig 16, following the pathindicated by the downwardly oriented arrows to the bottom of the well 13and the drill bit 14. As the drill bit 14 does its work, drill cuttingsare created, and these are picked up by the drilling mud and removed asindicated by the upwardly oriented arrows. From the top of the well 13,the solids-laden used drilling fluid is returned to the tank 11 where itmingles with the mud ingredients already there.

Viscometer 19 generates a signal sent through line 20 which is used tocontrol the speed or energy input of flow-controlled cavitation mixer 17as a function of viscosity. Viscometer 19 also generates a signal sentthrough line 21 which is used to control the introduction ofviscosity-modifying agent from source 22. A process controller, notshown, can manage the viscosity inputs and regulate the mixer and theviscosity-modifying agents according to programmed instructions.

It is thus not necessary to rely on the drill bit to perform the highlydesirable function of shear mixing. And, the drilling mud is at alltimes at the desired viscosity. The shear mixing action of thecavitation mixer 17 will be further explained with respect to FIG. 3.

FIG. 3 is a partly sectional view of a flow-controlled cavitation mixer,or FCCM. The FCCM comprises a substantially cylindrical rotor 31 withina housing having an inlet end 41, an outlet end 39, and encasement 33defining a cylindrical internal surface substantially concentric withthat of rotor 31. Rotor 31 is mounted on shaft 32 which is turned by amotor not shown. Shaft 32 is set on bearings 45 and 46 in extension 38,and its position may be adjusted horizontally (as depicted) to vary thespaces between rotor 31 and housing ends 41 and 39 as indicated by arrow47. Rotor 31 has cavities around its cylindrical surface; the cavitiesare illustrated as sections 34 a and as openings 34 b. Rotor 31 also hasa flow director 37 on its inlet side. While rotor 31 rotates, fluid froma source not shown enters through inlet 35 and encounters flow director37 which spreads it to the periphery of rotor 31 as indicated by thearrows. The fluid then passes through cavitation zone 40, a restrictedspace where cavitation is induced if the rotor is rotating fast enough,as explained elsewhere herein. Cavitation can be controlled to increasethe temperature of the fluid to a desired value by controlling the speedof rotation of the rotor. Conversely, energy input to the FCCM can becontrolled by direct measurement of rotation speed, a very useful datumto have for fluids of varying viscosity and rheology such as drillingmud.

The versatile FCCM is also able to act as a viscometer because, when itis not causing cavitation, it acts as a cylindrical spindle, a knownform of viscometer employing Couette principles. For the fluid materialsrelevant to the invention, viscosity may be expressed as the ratio ofshear stress to shear rate, or

$\mu = \frac{\tau}{\gamma}$

where the shear stress τ is

$\tau = \frac{T}{2\pi Rs2L}$

and shear rate

${\gamma = \frac{2\omega Rc2Rs2}{x2\left( {{Rc2} - {Rs2}} \right)}},$

that is 2ωR_(c) ²R_(s) ²/x²(R_(c) ²-R_(s) ²), where R_(c) is the radiusof the cyclinder, in this case the internal width of inlet and outletends 41 and 39, R_(s) is the radius of the spindle, in this case theradius of rotor 31, T is the torque of the rotor acting on the fluid, ωis the angular velocity of the rotor, and x is the radial location atwhich shear is being calculated. As indicated above, this formulaassumes there is no cavitation taking place around the rotor—that is,that the action of the cavitation mixer is limited to generating theshearing action that enables reading shear stress and shear rate withoutthe disruption that would be caused by cavitation. I call this the“shear mode,” and when the cavitation mixer is causing cavitation, Icall it the “cavitation mode.” The above described method of calculatingviscosity, and similar formulas in the literature using a spindle andcylinder, I call the “spindle viscosity formula” or, sometimes, “Couetteprinciples.”

Persons skilled in the art may observe that most presentations ofCouette principles or the cylindrical spindle measurement of viscosityillustrate a spindle that is longer than the diameter of the cylinder inwhich it resides, and that the cavitation mixer of the present inventionis illustrated as the opposite—that is, the length of the “spindle” isthe width of rotor 31, which is depicted as shorter than its diameter,or even its radius. This relationship of the cylinder and the housingwithin which it resides does not fundamentally change the calculation of

$\frac{\tau}{\gamma}$

to obtain the viscosity μ. However, some reports on the spindleviscosity formula are concerned with the effects of the space at the endof the spindle, and various workers have calculated additional formulasfor them. In the present invention, not only are relatively largesurfaces present on both “ends” of the rotor 31, but also, the fluidcontinually flows through the cavitation mixer while the calculationsare made. Although the non-cylindrical faces of rotor 31 (the “ends” ofthe “spindle”) are relatively large compared to the width of the rotor,their effects on the calculation of viscosity are reduced by twofeatures of the FCCM construction: first, flow director 37 spreads theincoming mud evenly over its surface so that when the mud enterscavitation zone 40 it will follow a helical path in substantiallylaminar flow over the cylindrical surface of rotor 31. In thenon-cavitation mode—that is, when the rotor 31 is not rotating fastenough to cause cavitation, the cavities 34 a and 34 b are neverthelessfilled with fluid which tends to remain in the cavities, providingsurfaces over which the fluid passes. As indicated in FIG. 3, theprofile of flow director 37 is a smooth curve tending to reduceturbulence and encourage laminar flow. The smooth curve profile of flowdirector 37 may be parabolic, elliptical, hyperbolic or a more complexsmooth curve, generally campanulate and asymptotic toward the neck ofrotor 31. Second, helical flow through cavitation zone 40, even in theabsence of cavitation, is somewhat assisted by the position of outlet 36near the periphery of rotor 31, as the mud passes quickly to outlet 36from cavitation zone 40 without establishing a significant flow patternon the outlet side of rotor 31.

Viscosity of slurries has been successfully measured in a helical flowinstrument. See, for example, T. J. Akroyd and Q. D. Nguyen, ContinuousRheometry for Industrial Slurries, 14^(th) Australasian Fluid MechanicsConference, 10-14 Dec. 2001. The authors recognized a tangentialcomponent to the shear stress as well as an axial component,incorporated into their calculations. See also Shackelford U.S. Pat. No.5,209,108. Because laminar flow is encouraged across the cavitation zonewhen measuring viscosity, pressure drop across the cavitation mixer maybe used, according to the classical Poiseuille formula explained below,to modify the calculation of viscosity.

In FIG. 4, a flow diagram is presented for a loop of the invention. Inthis configuration, the cavitation mixer 53 performs two separatefunctions. In one function, it is operated with power input sufficientto cause cavitation in the fluid until a desired temperature is attainedin the fluid. In the cavitation mixer's second function, the power inputis reduced so that no cavitation takes place and the cavitation mixeracts as a viscometer.

In the optional “straight-through” mode, which does not employ therecycle loop, the drilling mud ingredients pass through valve 51 onconduit 50 to pump 52, through valve 62, and then into cavitation mixer53, where they are heated and mixed, then through conduit 54 to Coriolismeter 55 and viscosity meter 61 before passing through valve 56 to awell, or to storage or other use not shown. Coriolis meter 55 (which maybe an E+H Coriolis meter) may measure density in conduit 54. Viscositymeter 61, which may be a Brookfield TT-100 viscometer, may be programmedto continually read viscosity at all Fann 35 speeds.

But an important feature of the invention is that an aliquot of fluid(drilling mud) can be isolated in the loop defined by closing valves 51and 56 and opening valves 58 and 59, thus flowing an isolated, knownquantity of fluid continuously in the loop through cavitation mixer 53,conduit 54, conduit 57 and again through conduit 54 to cavitation mixer53. This may be referred to as the “loop mode.” In accordance with theinvention, the cavitation mixer is operated in the cavitation mode toquickly heat the mud aliquot to a desired temperature (measured by atransducer or other device not shown), and then it is operated in thenon-cavitation, or shear, mode so it can shear the aliquot and beutilized as a viscometer. Acting on the same aliquot of drilling mud asit circulates in the loop, the cavitation mixer 53 may be programmed toheat the mud, by cavitation, to a second temperature and then, withoutcavitation, to shear it. While shearing the mud, the cavitation mixermay be utilized as a viscometer employing Couette principles. Theisolated aliquot may be further heated to a third temperature andviscosity measurements obtained as described elsewhere herein, as afunction of torque on the mixer's shaft and angular velocity of therotor.

When viscosity-modifying agents or other chemicals are to be added tothe mud, valve 62 may be closed and valves 64 and 70 opened, causing mudto flow through additive conduit 65. Additive conduit 65 passes throughan eductor 67 which assists the feeding of dry chemical (such as drypolymer) from hopper 66 if such a feed is required by the controller.Conduit 65 also is associated with liquid feeder 68, which can, oncommand, deliver doses of liquid chemical (such as dissolved polymer)into additive conduit 65 through inlet 69. Additives introduced to themud in additive conduit 65 will be thoroughly mixed into the mud when itpasses into cavitation mixer 53.

Persons skilled in the art may recognize that additive conduit 65 is notessential for liquid feeder 68, which could be placed on conduit 50anywhere upstream of cavitation mixer 53. Eductor 67 for solidadditives, however, is an in-line device and accordingly is best used ina separate conduit such as additive conduit 65.

A dashed-line rectangle bearing the reference number 63 on conduit 57 inFIG. 4 is labeled “Mud Check Instruments.” This represents any or all ofmeters, probes, instruments and transducers for detecting or measuringdensity, flow, viscosity, pH, percent solids, water cut or oil/waterratio, electrical stability, particle size, temperature and otherproperties of the mud. Such devices are not limited to positioning in oron conduit 57. They may be anywhere in the system; for example,temperature probe 71 and pressure probe 72 are illustrated in conduit54. Included in Mud Check Instruments 63 are (one or more) computers,processors or controllers necessary or useful to monitor and modify theproperties of the mud in the loop. For example, computers, processors,or controllers may be programmed to vary the power input and/or angularvelocity of the shaft of cavitation mixer 53, or to open and closevalves so that hopper 66 or liquid feeder 68 can deliver prescribedamounts of additives. Data about the mud and the well's operation may beaccumulated to provide increasingly accurate refinements to be usedpossibly in the “straight-through” mode. Additives are proportioned tothe aliquot in the loop and circulated to confirm the modifications madeto its properties. The “straight-through” mode may be modified to takethe illustrated detour through additive conduit 65 for continuousproportionate injections of additive(s).

Viscosity may be measured by a viscometer, not shown, in conduit 54 orconduit 57. Optionally, viscosity may be read by pressure difference asis known in the art. The reduction in pressure between points Pr1 andPr2 may be ascertained by any acceptable pressure reading devices andthe difference used to reinforce the calculations according to thespindle viscosity formula described above and/or viscometer 61.Poiseuille's pressure drop equation for viscosity μ for a fluid flowingin a tube is:

$\mu = \frac{\pi R^{4}{{gc}\left( {P_{1} - P_{2}} \right)}C}{8{LQ}}$

where R is the radius of the tube, gc is the gravitational constant, P₁is the measured upstream pressure in the tube, P₂ is the measureddownstream pressure in the tube, C is a constant conversion factor forexpressing viscosity in poises, L is the distance on the tube between P₁and P₂, and Q is the flow rate of the fluid in the tube. So, where theradius of the tube is fixed and the flow is steady, and becauseeverything else is a constant except the measured pressures, theviscosity μ is directly proportional to the pressure difference.

One of the advantages of my process is that data may quickly beaccumulated for more than one temperature for one or more aliquots ofthe mud. The aliquot isolated in the loop is easily ramped up from, forexample, 100° F. to 150° F. to 175° F. In this example, the aliquot isfirst heated by the cavitation mixer in the cavitation mode to 100° F.,the viscosity is measured either by Couette principles applied to thecavitation mixer or by a separate viscometer, or both, then the mud isheated to 150° F. and the viscosity is again measured by one or moredevices, and the mud is further heated by the cavitation mixer to, say,175° F., after which the viscosity is again measured by at least onedevice, which may be the cavitation mixer itself. Additional temperaturelevels may be included, or not. As Couette principles require inputs oftorque and angular velocity of the rotor 53, these are monitored andsent to the process controller along with the temperature and otherproperties.

Thus, whether viscosity is measured in the loop at one temperature or atmore than one temperature, the viscosity measurements can be stored(along with any other properties found by other instruments) and thenused in the straight-through mode to heat the fluid and adjust theviscosity to the desired value until it is determined that additionaldata are needed. Converting from the loop mode to the straight throughmode may be accomplished either by the programmed controller or by ahuman operator.

1. Method of measuring viscosity of a fluid at a desired temperature comprising (a) heating said fluid in a cavitation mixer to said desired temperature and (b) calculating viscosity of said fluid by a spindle viscosity formula applied to said fluid in said cavitation mixer.
 2. (canceled)
 3. Method of claim 1 wherein said cavitation mixer is a flow-directed cavitation mixer.
 4. Method of preparing drilling mud ingredients for use in a well comprising (a) pumping said drilling mud ingredients from at least one source through a conduit, said conduit including a recycle loop with valves for isolating an aliquot of said drilling mud ingredients in said recycle loop, (b) passing an aliquot of drilling mud ingredients so isolated through a cavitation mixer located on said recycle loop to heat, by cavitation, said drilling mud ingredients to a desired temperature, (c) passing said aliquot of drilling mud ingredients through said cavitation mixer, without cavitation, to shear said drilling mud ingredients at said desired temperature to obtain a desired viscosity, thus making a prepared drilling mud for a well, and (d) passing said prepared drilling mud outside said loop for storage or use in a well.
 5. Method of claim 4 wherein said cavitation mixer is a flow-controlled cavitation mixer.
 6. Method of claim 4 including, in step (c), determining said desired viscosity by Couette principles, optionally modified by a function of a difference in pressure across said cavitation mixer, applied to said cavitation mixer.
 7. Method of claim 4 including, in step (c), measuring viscosity by an in-line viscometer within said loop and regulating, as a function of said viscosity, at least one of (i) power input to said cavitation mixer, and (ii) the addition of a viscosity modifier to said drilling mud ingredients.
 8. Method of claim 4 including intermittently or continuously monitoring said drilling mud ingredients in said loop for at least one of density, flow, viscosity, pH, percent solids, water cut or oil/water ratio, electrical stability, particle size, and temperature. 9-35. (canceled)
 36. Method of measuring the viscosity of a fluid at a desired temperature comprising (a) heating said fluid to said desired temperature in a cavitation mixer having a rotor, said cavitation mixer operated in the cavitation mode and (b) calculating viscosity of said fluid by a spindle velocity formula applied to said rotor in said cavitation mixer operated in the shear mode to shear said fluid at said desired temperature.
 37. Method of claim 36 wherein said cavitation mixer is a flow-directed cavitation mixer.
 38. Method of claim 36 wherein said fluid is an aliquot of a drilling fluid isolated in a loop including said cavitation mixer and a pump.
 39. Method of claim 38 including using said calculated velocity as a factor in controlling viscosity of said drilling fluid as drilling is conducted in a well.
 40. Method of claim 38 including additionally measuring viscosity of said aliquot of drilling fluid by a viscometer.
 41. Method of claim 36 wherein said spindle velocity formula includes a function of torque and angular velocity of said rotor. 42-55. (canceled) 